Hydrocarbon conversion process



April 14, 1959 J. w. .JEwELL HYDRocARBoN CONVERSION PRocEss Filed Dec. 9, 1955 ATTOR N E YS United States atent HYDRDCARBON CONVERSION PROCESS Joseph W. Jewell, Summit, NJ., assiguor to 'Ihe M. W. Kellogg Company, Jersey City, NJ., a corporation of Delaware Application December 9, 1953, Serial No. 397,160

17 Claims. (Cl. 208-74) This invention relates to an improved method of processing residual oils for the production of gasoline, and more particularly, it pertains to the catalytic cracking of residual oils for conversion to a gas oil product which is especially suited as feed stock to a second catalytic cracking treatment in which a gasoline of high anti-knock quality is obtained.

In conventional catalytic cracking operations a residual oil is not charged as the sole feed material for direct production of gasoline, because experience has shown that residual oils, in general, contain an undesirable quantity of metal contaminants which seriously reduce the life of cracking catalysts. Further, residual oils usually contain carbon residue in excess of 0.6% by weight, and this value is recognized at the present time as the limit above which coke production becomes uneconomical for opera- 2,882,218 Patented Apr. 14, .1959

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rates this quantity is materially increased and since it appears that high coke on catalyst serves to enhance or catalyze further coke production, it follows that the initial gas oil produced from cracking heavy components of the residual oil forms more coke upon further cracking than if gas oil so produced had been charged as the initial feed material to an operation in which coke on catalyst could be kept at a low level. The same theory of operation appears to apply to gas production. Hence, in the operations practiced today, for a given amount of gasoline produced by catalytic cracking of residual oils there may result, a higher yield of coke and normally gaseous product material than the method of the present invention. Further, the cost of catalyst replacement in heretofore methods of processing residual oils may be greater than in the method of this invention.

An object of this invention is to provide an economical and effective method of processing residual oils for ultimate gasoline production.

Another object of this invention is to process residual oils under conditions suitable for the production of feed preparation of superior gas oil feed stock for the further tions involving the direct production of gasoline at conversion levels in the order of about 40 or 45 to about 65. As a result, in the majority of catalytic cracking operations of the type described above, gas oil is the sole feed material or a mixture of gas oil and a limited amount of residual oil is used.

Residual oil is a distress stock in the petroleum industry, because, at present, a satisfactory method of processing this material economically to a valuable product is not known. A substantial amount of the residual oil is sold as fuel oil, which is significantlyA less profitable than the sale of gasoline. Extensive effort is being made to process residual oil for the production of more valuable products, however, the characteristics of this material present many difliculties for commercial exploitation. As previously indicated, residual oils contain large quantities -of components having high coke forming tendencies and Yalso metal in the combined form which exerts adverse effects on the selectivity and activity of cracking catalysts.

'The processing of these materials in cracking operations .fat conversion levels of 40 or more has been found not too satisfactory, and the main justification for this operation is the lack of a better treatment for gasoline production. Careful study of this problem led to the conclusion that one of the major reasons for excessive coke formation from residual oils at high conversion rates was the unavoidable difference between the average and maximum time of contact in a catalytic reactor. This led to the further conclusion that if a reactor were designed to minimize this difference, and if the conversion level were lowered to reduce the required average time, that a large yield of gas oil suitable for further catalytic cracking could be produced without excessive coke formation. A small fraction of unreacted oil, if desired, could be yrejected as unsuitable for catalytic cracking, and this Ifraction would contain a high percentage of the carbon residue and metals in the residual feed. Furthermore, any gasoline made in this stage would be a much better quality than could be produced by coking or thermal visbreaking of the residual feed. When residual oil is subjected to catalytic cracking there is a relatively high deposition of coke on the catalyst. At high conversion production of gasoline by catalytic cracking.

Other objects and advantages of this invention will become apparent from the following description and explanation thereof.

It is contemplated by means of this invention to first subject a residual oil to contact with a cracking catalyst under cracking conditions to produce a gas oil product at a conversion level of about 5 to about 20%, and subjecting the gas oil thus produced to a cracking catalyst under cracking conditions to produce a gasoline product at a conversion level of at least about 35%.

In catalytic cracking operations, the conversion level serves adequately to describe the severity of the operation. The severity desired may be attained by regulating the temperature, the time of contact, the catalyst to oil ratio or the level of catalyst activity. It is the relative effect of these variables with which we are concerned in accomplishing our invention, particularly, the effect of the rate of catalyst circulation and the level of catalyst activity. The best method available for indicating severity at present, is to describe a cracking operation by reference to the conversion level, which is defined by the formula:

Vf-V

Va wherein Vf is the volume of feed boiling above 400 F., Vp is the volume of product boiling above 400 F. and Vo is the volume of feed.

In the mild treatment of residual oil, in accordance with this invention, the conversion level is maintained not higher than about 20%. The residual oil to be used in this treatment may or may not contain gas oil components which can be satisfactorily processed in the severe cracking operations. If the feed does contain such gas oil components the operating conditions will be such as to minimize conversion of such gas oil. It is preferred that the residual oil contain substantially all of the metal contaminants and carbon residue of the crude oil, and that any material which can be separated therefrom, substantially free of metal contaminants and carbon residue Percent conversion 1 00 X should not be included as part of the residual oil feed to the mild cracking step. Hence, the mild cracking step is of such severity that substantially all of the metal con- In in-l stances where met-al contamination of residual oil is extremely high, -it `may be desirable to discard a-portion of the liquid product boiling higher than gas oil from the mild cracking operation in order to avoid an unduly high rate of catalyst deactivation. The primary cracking reactions in the mild treatment involve essentially the cracking of components in ythe residual oil boiling range to products in the gas oil boiling range with consequent deposition of metal contaminants and carbon on the catalyst and the production of a gas oil suitable as feed -for severe cracking to gasoline, and with the practical minimum production of gasoline. It should be recognized that any gasoline so produced will be much superior to gasoline produced in a thermal vis-breaking operation.

The mild cracking operation ,is conducted at a temperature of about 700 F. to about 1000* F., more usually, about 800 to about 900@ F. The pressure used can'be at Aatmospheric `level or it can be as high as 50 p.s.i.g. Higher -superatmospheric pressures are avoided only to `the extent required by the problem of the degree of vaporization necessary to distribute the Vfeed on the flowing catalyst. Within the scope of the present invention, this problem can be substantially eliminated within economic limits by injecting gaseous materials into the cracking zone for the purpose of decreasing the partial pressure of the residual feed, and so promote its vaporization. For this purpose, steam can be used, which can be produced by utilizing the heat of combustion in the regeneration zone of the system. Another gaseous material which can be used is hydrogen, and it can also serve to suppress the adverse eifects of metal contaminants on catalyst life and/or to reduce carbon or coke formation. The normally gaseous hydrocarbon product from severe gas oil cracking which contains hydrogen in adrnixture with C1-C3 paratlins or olens, can be used. In some instances, the flue gas from the regeneration zone might be used for this purpose.

The quantity of residual oil processed relative to the amount of catalyst contacted therewith is measured in terms of the weight space velocity, which is defined as the pounds of oil feed charged to the reaction zone on an hourly basis per pound of catalyst present therein.

The weight space velocity can vary over a wide range,

namely about 0.1 to about 15, however, more usually, it will be about 1 to about l0. In either a fluid bed or a moving bed system, the catalyst to oil ratio is measured as the rate of catalyst being circulated to the oil feed rate, on a weight basis. Generally, for the mild cracking operation, the catalyst to oil ratio can vary from about 0.1 to 20, more usually, about 0.5 to 5.0. Further, the residual oil feed can be preheated under suitable conditions, which may include mild thermal cracking, in order to reduce Athe catalystoil ratio required yfor satisfactory adsorption of the feed on the catalyst.

The catalyst employed for the mild cracking `operation can be any suitable cracking catalyst such as, `for example, a silica containing cracking catalyst or a siliceous catalyst commonly used for this purpose in known cracking operations. The silica containing cracking catalyst can have about l0 to about l00%,slica, based on the total weight of the catalyst, although more usually, silica comprises the majority but not all of the catalytic material, for example, in amounts of about 15 to about `95% by weight of the total catalyst. `Specific examples of cracking catalyst which can be used for the purposes of mild cracking in the present invention are bauxite, silica gel, Superiiltrol, various types of clays, silica-alumina, silica-magnesia, silica-zirconia, silica-boria or mixturesof the foregoing. 'The synthetically prepared silica containing gell catalysts such as, for example, silica-alumina, silicamagnesia, etc., in which Vsilica comprises about 70 to about 90% by weight of the total catalyst, are specially suited for the mild cracking operation. The `cracking catalyst described above is prepared by various methods well-known in the art. Therefore, it is not necessary to furnish a description herein. When hydrogen is used as a diluent gas, it may be desirable to incorporate a hydrogenating component in the cracking catalyst. Suitable hydrogenation components for this purpose lare metals of group VIII, compounds of the left-hand elements of groups V and VI, etc. More particularly, the hydrogenating component may be nickel, cobalt, iron, platinum and palladium and/ or the oxides and/ or sullides thereof; the oxides and/or suldes of molybdenum, chromium, vanadium and tungsten, etc. Specific examples of these hydrogenating components are molybdenum trioXide, chromium oxide, tungsten oxide, tungsten sulfide, nickel oxide, cobalt oxide, etc. When a hydrogenating component is used in combination with the cracking catalyst, generally, it constitutes 0.1 to about 15%, based on the total weight of the catalyst. The activity of the cracking catalyst is based on a D-l-L lactivity rating which is described in an article by R. V. Shankland and G. E. Schmitkons published in Ind. Eng. Chem., vol. 39, 1947, page 1138, and in the report of the v27th annual meeting ofthe American Petroleum Institute, November 1947.

In the mild cracking operation, the catalyst due to metal contamination becomes permanently deactivated and, hence, requires replacement at such a rate as to maintain a desirable D-,LL activity. Generally, for this purpose, the average activity of the catalyst in the mild cracking `zone is maintained at about l0 to about 25 D+L. The catalyst replacement to maintain this activity is preferably effected by using, as makeup, catalyst which is Withdrawn from the severe cracking zone. ln the severe cracking zone the average activity is maintained at about 20 to about `45 D+L. In the preferred operation, freshly prepared or new cracking catalyst having an activity of about 40 to about 70 D-l-L is charged to the severe cracking operation in an amount sufficient to provide catalyst replacement for both mild and severe cracking operations. It is preferred to use catalyst having a lower D--L activity than fresh catalyst in the mild cracking operation, because there is a tendency for overcracking to occur when highly active catalyst comes into contact with a residual oil stock, A high catalyst replacement rate for the mild cracking operation will result in the maintenance of a higher average D+L activity in a severe cracking operation than is considered cconomical rin a conventional system. For the purposes of this invention, it is preferred to maintain an average D-l-L activity of about 30 to about 36in the severe cracking zone. In conventional operations, it is customary to maintain an average D-l-L activity of about 23 to 26 for the production of gasoline. The maintenance of a higher activity level for the severe cracking operation results in several advantages. lIn one instance, a better product distribution may be obtained by the use of a higher activity level. In another case Where a given product distribution `is desired, the higher activity lcvel'can be used to reduce the required reaction temperature. Such rcduction of temperature will require less heat from the regenerator and thus reduce the C/ O ratio, which in turn improves product distribution, or to reduce the rcgenerator temperature which improves ycatalyst life. For a given performance, another method of compensating for the increase in activity level is to reduce the quantity of catalyst in the reaction zone and, hence, a smaller size reaction vessel can be used, and a consequent higher space velocity.

When the average D-l-L activity of catalyst in the severe cracking operation is maintained at about 30 to about 36, it is preferred to maintain an average D-l-L catalyst activity in the mild cracking zone of 'about l0 to about 20. In essence, the circulation of catalyst `in the entire system from the standpoint of Areplacement involves withdrawing catalyst from the mild cracking zone to discard the same from the system at Aa'rate sufficient to maintain the desired activity level therein. The loss of :inventory .ia thel mild cracking zone is made :up by utilizing catalyst which is withdrawn fromv the severe cracking operation, and in turn, the catalyst inventory in the severe cracking zone is maintained by charging thereto fresh or new catalyst at the same rate. It is preferred that the catalyst used as replacement for the mild cracking operation is withdrawn from the regeneration system of the severe cracking operation.

As mentioned above, the catalyst in the mild cracking operation becomes contaminated with metals and these metals are undesirable in their effect because they shorten the life of the catalyst and can have an adverse effect upon product distribution. In order to offset at least in part this undesirable effect of metal contamination, it is contemplated using all the product vapor from severe cracking or at least that portion which is normally a dry gas product.A This gas containing a considerable portion of hydrogen in admixture with C1-C3 hydrocarbons can be used in place of steam to reduce the partial pressure of the oil feed. With less steam the overall deactivation of catalyst may be reduced. The high partial pressure of excess hydrogen and light hydrocarbons may also serve to suppress carbon formation.

As a result of the mild cracking operation, carbonaceous material is deposited on the cracking catalyst and in order to restore the temporary reactivating effects of this deposit, the catalyst may be subjected to a regeneration treatment by means of an oxygen containing gas such as, for example, air, diluted air containing about 2 to about by volume of oxygen, etc., and at a temperature in the order of about 800 to about '1250" F., more usually, about 950 to about 1050 F. The cracking operation is endothermic in nature, consequently, heat must be supplied in order to maintain the desired temperature. It is known that a substantial amount of the heat can be supplied by circulating catalyst at a suliicient rate such that the sensible heat above reactor temperature level contained in the catalyst can be utilized for furnishing the required endothermic heat of reaction in the cracking zone. This principle can be used in the present invention in order to attain what is commonly referred to as a heat balance system. However, it is also contemplated preheating the feed, using lower catalyst to oil ratios in the order of about 1 to about 5 and utilizing the heat of combustion in the regeneration zone for the purpose of generating steam. As a result of producing a minimum yield of gasoline product in the mild cracking operation and selecting conditions so as to selectively crack compounds boiling in the residual oil range to gas oil components, the endothermic heat requirement of the reaction is significantly less than is normally required for a conventional cracking operation. In view thereof, by using vthe preheating step for the residual oil feed to the cracking operation, a lower catalyst to oil ratio can be used effectively to improve product distribution as well as to reduce any loss of product materials being burned in the regenerator. Further the heat of combustion can be used for heating a diluent gas such as, for example, hydrogen containing gas, normally gaseous hydrocarbons, etc., and the heated gaseous materials can be fed to the mild cracking zone for the dual purpose of reducing the partial pressure of the residual oil and supplying at least part of the endothermic heat of reaction.

Except for the increased level of catalyst activity the severe cracking step of this invention is operated in the same manner as is commonly practiced in the art today. In this type of an operation, the feed material consists primarily of a gas oil having an initial boiling point in the order of about 350 to about 450 F. and an end point of about 850 to about l000 F. or higher. The feed material can consist entirely of the gas oil which is produced from the mild cracking operation, or it can be a mixture of the gas oilv product from the mild cracking step and a straight run gas oil. As a result of the economic limits ofconversion in the severe cracking operation the product contains a gas oil fraction and this is ordinarily referred to as cycle oil. This product material may or may not be recycled to the severe cracking zone, and if not vso used, it is withdrawn for use outside the unit. This operation is similar to the type which is in practice at the present time in some commercial operations. For the purpose of this invention, the severe operation is designed to produce primarily a gasoline product. The severity, in terms of conversion level, is at least about 35% up to about 70%, although, more usually, it is about 45 to about 65%. The operating conditions which are used in this part of the process fall essentially within the same range as described hereinabove for the mild cracking step, however, the conditions are selected on the basis of selecting those variables whose combined effect is severe in nature. The method of effecting a severe operation is well-known to those skilled in the art, hence, it is not necessary to furnish details herein. In general, the temperature for the severe cracking step is about 800 to about 1050 F., more usually, about 900 to about l000 F. The pressure under which this reaction is eected can be atan atmospheric level or up to about 50 p.s.i.g. Usually, the lower pressure levels are preferred such as, for example, 10 to about 20 p.s.i.g. The weight space velocity as defined hereinabove varies from about 0.1 to about 10, more usually, about 0.5 to about 5. In the severe cracking operation, the catalyst to oil ratio can be varied considerably over a wide range of about 0.5 to about 25, however, more usually, a catalyst to oil ratio of about 2 to about 10 is used.

The catalyst to be used in the severe cracking step can be any one or more of those described hereinabove in connection with the mild cracking operation. In commercial practice, however, the most widely used catalysts are either Superfiltrol or synthetic silica-alumina catalyst. Superfiltrol is an acid treated Bentonite clay which is refered to as a natural catalyst; whereas the synthetic silica-alumina lis prepared by methods well known to those skilled in the art from various chemical compounds as starting materials. In some commercial operations, mixtures of silica-magnesia and silica-alumina have been used in order to obtain improved selectivity or higher liquid production. However, silica-magnesia results in a lower octane product, consequently, its use depends upon the result desired for this type of operation. It was noted hereinabove in connection with the catalyst to be used in the mild cracking step that the discarded catalyst from the severe cracking zone can be used for the mild cracking operation. This technique has its obvious economical advantages, however, in mild cracking operations where large quantities of steam are employed for the purposes indicated above, it may be desirable to employ Superfiltrol as the fresh cracking catalyst rather than synthetic silica-alumina by reason of its greater stability towards steam. Where steam stability is important, the effect can also be had by using silica-magnesia or a mixture of silica-magnesia and silica-alumina in the severe cracking operation and supplying the discarded catalyst to the mild cracking operation. Silica-magnesia has a significantly greater stability towards steam than silicaalumina. Consequently, it would be expected that the activity of the catalyst in the mild cracking zone would decline less rapidly.

The residual oil to be processed in the present invention can be a reduced crude which constitutes about 20 to about 50% of the total crude and has an API gravity of about 10 to about 25. Reduced crudes can be readily used-in'uid catalytic cracking systems without taking extreme measures to insure complete vaporizat-ion of the feed material to avoid defiuidization of the reaction bed through wetting of catalyst particles. Generally, these reduced crudes contain about 30 to about 90% of components boiling above 700 F. Those materials which boil above 700 F. are relatively easily cracked to coke and normally vaporized products and, hence, the quantity of these materials in a feed stock can serve to indicatey qualitatively the foraokahility or susoeptility to .oraolsing of such materials. There are also present in reduced crueles compounds of a highly asphaltic nature, sulfur .Containing compounds and salts. Q-metals which have an adverse effect upon cracking catalyst activity. In general, the carbon residue of a reduced vcrude is at least 1% by weight, more usually, about 2.5 to about 30% by weight. The sulfur content may vary widely depending uponv the source from which the crude oil is obtained. In general, the sulfur content of reduced crude is at least 0.1% by weight, more usually, about .2 to about by weight. The metal contaminants in the reduced crude are meas-A ured. in terms. of a so-ealled uielsel equivalent. in parts per million. noma and this serves to indicate the. total. concentration" of metal contaminants on .au equivalent. nickel. basis The metals normally .found iu reduced @rudes aud which cause adverse eleets ou cracking cata-V lyst activity are, for example, stelsel, copper, iron, vanadi-A um and cobalt.. Since uiekel. Ycauses the greatest otnouut. ot deactivation of the metal Contaminants, the .nickel equivalent basis has been used. to designate the combined effect of the concentration of these Contaminants. Gen-v erally, in reduced crudes, the nickel equivalent may be at least about 0.5 ppm., and when the metal contaminants constitute at least 2 p.p.m. on a nickel equivalent basis, there is a significant effect upon cracking catalyst activity. It is found, however, that a large number of reduced crudes contain about 5 to about 70 p.p.n1. of metal contaminants on the nickel equivalent basis. Thev preferred feed stocks for the mild cracking operation contain essentially all of the metal contaminants of the crude oil and a quantity of the carbon residue such that the gas oil employed as feed stock for the severe cracking operation contains not more than about 0.4 to 0.8% carbon residue. When the gas oil is the sole feed to the severe cracking operation the carbon residue does not exceed about 0.6%. The heavy residual oils can be used as feed to the mild cracking operation. The heavier residual oils are, for example, vacuum tars, residual tars, fuel oils, etc., which have API gravities in the range of about 1 to about 12.. These stocks present diiculties in fluid systems by reason of the high boiling nature of the compounds making up the material. A Llarge part or all of such heavy residual oils are liquid at the temperatures used in the mild cracking operation. The ease with which the compounds in these heavy residual oils are cracked, is an important factor in establishing conditions for the operation in which they are used. Since these stocks are eraeked easily and they are of. a high boiling nature., c are is taken in determining the quantity of catalyst for contact therewith at the point of charging the liquid feed to the reaction -zonel with large quantitiesv of diluent gas, eg., steam, in order to reduce the partial pressure of the oil and thus to increase the degree of vaporization` and the. ease of distribution thereof on the catalyst surface.

The metal concentration, sulfur and carbon residue of these heavy residual oils is on the average higher than reduced crudes, although the amounts will fall within the ranges specified hereinabove for the reduced crudas. With respect to the boiling characteristics of the heavy residual oils, for the purposes of this specification and the appended claims, such materials have an initial boiling point of about 850 to about 1000" F. Since the heavy residual oil comprises the heaviest fraction of the total crude, it is not necessary to specify the end point by reason that the boiling point of theA heaviest compound in total crude will be representative of the end point of the heavy residual oil.. lt should be understood that the ter-rn "resitlual oil." as. used for this specification and the appended claims, includes reduced crude and heavy residual oils.

The separation of suitable feed material, from the reduced crude for the severeA cracking operation can be accomplished by distillation under vacuum or other suitable separation means. In one embodiment, crude oil is topped or separated by suitable means, e.g., distillation, to separate therefrom straight run gas oil. The straight run gas oil is charged to the severe cracking operation. The reduced crude from the topping operation may be further subjected to distillation under vacuum to separate a gas oil fraction which is suitable for severe cracking and a residual oil fraction. The gas oil fraction from the vacuum distillation operation is charged to the severe cracking operation. The residual oil fraction is charged to the mild cracking zone, and a gas oil product is obtained which is charged to the severe cracking operation. In one aspect of this invention the reaction product from the mild cracking zone can. .be passed to the crude oil topping unit for separation ofthe gas .oil Product with the Straight run gas oil.. The total feed to the severe cracking operation .should .have less than about 2 noto of metals, nickel equivalent, preferably less .than about l ppm of metals. nickel equivalent.

The catalyst Particles for the fluid system of this invehtiou will have. a size ranging from about 0 to about 250 microns, more usually, about 10 to about 100 microus. The gaseous materials being passed upwardly in the reaction, regeneration and stripping zones, will have a superficial linear gas velocity in the range of about 0.1 to about 50 feet per second. For commercial practices, the superficial linear gas velocity of the. upllowing materials is usually about 1 to about 2.5 feet per second in the processing zones of a bottom drawoff phase system. It is also contemplated involving an upow type of system in which the superficial linear gas velocity is sucient to carry overhead all of the finely divided catalytic material which is introduced into the processing zone. I n the upow system, the superficial linear gas velocities in the processing zones are about 2 to about 10 feet per second. The use of heavy residual oil as feed material to the rnild cracking zone may advantageously employ the upflow technique in order to avoid defluidization effects. The upflow system .is referred to, generally, as the high velocity method, and. it has the advantage of providing a more uniform residence or contact time between the catalyst particles and the gaseous material.

In order to provide a better understanding of the present invention, reference will be had to the accompanying drawing which forms a part of this specification.

In the figure, reduced crude containing a fraction of clean gas oil is heated to a suitable temperature and is charged from line to the lower part of fractionating tower 10.1. In fractionating tower 101, the. portion of reduced crude which consists of gas oil substantiallyI free of metals and carbon residue is yielded overhead through a line 10,2; whereas the heavy residual oil is separated and yielded as a bottom fraction through a line 103.

The gas oil which is separated from the reduced crude in tower 101 is combined with heavy recycle gas oil which is supplied by means of line 190, and with light recycle gas oil which is supplied by line 192. The heavy and light recycle gas oil streams are withdrawn from tower 113 as Side stream products and theyy are divided such that part of each stream may be passed to storage for further treatment and the remaining p ait is. combined with the virgin gas oil from line 102. It should be noted that the heavy and light gas oil side streams from fractionator 113 are composed partly of refractory cycle gas o il from severe gas oil cracking and partly of product gas oil from the mild cracking step. Thegas oil produced overhead from tower 101 is combinedv with the light and heavy gas oil fractions from fractionator 113 and the total material is charged to a furnace 200 wherein it preheated to a temperature of 600900 F. The preheated feed material is discharged fromI furnace 200 by means of a line 202, and thence, it is fed into a transfer line 204- wherein it picks up regenerated catalystflowing from standpipe 206, containing a yslide valve 208, and transporting the same to an upilow reactor 210. Steam may be injected in line 202 through line 260, if desired. In the upow reactor, the temperature is maintained at about 900-950 F., at a total pressure of 15-16 p.s.i.g., the catalyst to oil ratio is from 3 to 10:1, and the quantity of catalyst which is present in the reactor 210 relative to the oil feed provides a space velocity of 0.5-5 Vo/hn/Vc. Under the conditions existing in this reactor, 55-65% of conversion is effected. The supercial linear gas velocity of the reactant materials is 2.6-6.0 feet per second, and it is sucient to carry overhead from the reactor 210 all of the catalytic material being supplied through transfer line 204. The vaporous reaction product and spent catalyst are removed from the reactor 210 by-means of a line 212, and it is transferredto the upper part of a reactor hopper 214. A spent catalyst ,bed E is situated in reactor hopper 214, vand it has a level 216. Stripping steam is introduced from line 218 to the bottom part of reactor hopper 214 at the rate of 1 to 5 volumes of steam per volume of uidized catalyst being circulated. Bed E in reactor hopper 214 is maintained in a dense phase, and at a temperature of about 900-1050 F. The vaporous reaction product is removed from reactor hopper 214, and it flows into line 121, wherein it is mixed with the tar fraction of the reduced crude for processing in reactor 1-26. Spent catalyst is withdrawn from reactor hopper 214 by means of standpipe 220, in which there is installed a slide valve 222. Air is supplied from a main source 224, and it passes through line 226, which serves to pick up catalyst owing from standpipe 220, and transport the same through transfer line 228 to the upper part of regenerator 230. Flue gas is withdrawn from regenerator 230 by means of a line 232, and any catalyst which is entrained therewith is separated and recovered by means of cyclones, etc., not shown. In the bottom part of regenerator 230, there is situated a grid plate 234, upon which regenerated catalyst bed F is supported. Air is supplied for the purpose of regeneration through line 236 and it passes upwardly through the regenerator by means of line 238. Regeneration of catalyst is effected at a temperature of about 1050" F. and at a total pressure of about l5 p.s.i.g. The spent catalyst contains a carbonaceous content of 1.5-2.5% by weight and by means of regeneration in vessel 230, the carbonaceous content is reduced to about 0.5% by weight. The temperature of regeneration is controlled by withdrawing regeneratedcatalyst from bed F by means of a standpipe 242, containing a slide valve 244, and transferring the same by means of air which is supplied through line 246 and by means of branch line 248. The regenerated catalyst suspended in air is passed from line 248 through a catalyst cooler 250 wherein the temperature is reduced t`o about 850 F., and it is returned to the regenerator below grid plate 234 by means of line 254.

' The tar fraction from tower 101 is charged to a furnace 105, wherein the temperature of the tar is raised to about 800 F. The preheated tar fraction is discharged from furnace 105 through a line 107, and thence it flows into line 109 in which there is flowing a heavy oil consisting of slurry withdrawn from the bottom of fractionator 113, by means of line 111, and containing catalyst'fines commingled therewith and, optionally, other heavy gas oil which is unsuitable for severe cracking and which may bel Withdrawn from fractionator 113 through lines 138 and 140. The purpose of recycling all ora part of this additional gas oil through the mild crack-v ing zone is .to improve its quality as to metal content and/or carbon residue, thus to make it suitable for the severe cracking operation.

The liquid feed material in line 109 is combined with all or part of the reaction product from a severe catalytic cracking operation which isv supplied froma line 121..

4.'10 The vaporous reaction product and liquid 'feed material are passed as a combined stream in a line 120. Freshly regenerated catalyst from regenerator 164 is supplied from a standpipe 123, containing a slide valve 124, and it flows into line for admixture with the feed material. The mixing of vaporous reaction product with liquid feed serves to vaporize a substantial part of the liquid feed, so that upon mixing with catalyst, uniform absorption of oil on the catalyst is accomplished. The total feed material and regenerated catalyst flows from transfer line 120 into an upflow reactor 126. In reactor 126, the temperature is maintained at about 825 F. and at a total pressure of 12 p.s.i.g. The rate of catalyst being charged to reactor 126 relative to the rate of oil feed on a weight basis, is about 3 to 1. The superficial linear velocity of the reactant materials passing upwardly through reactor 126 is about 3-4 feet per second, and it provides a fluid density of 8-10 pounds per cubic foot. The quantity of catalyst which is present in reactor 126 relative to the oil rate provides a space velocity of 1-10 Vo/hr./Vc. As a result of the conditions existing in reactor 126, 3-10% conversion of feed is elected therein. Catalyst and the product resulting in reactor 126 are withdrawn overhead through a line 127 and discharged to a reactor hopper 129. A iluid catalyst bed C is maintained in reactor hopper 129, and it has a level 130. In reactor hopper 129, any heavy oil which is adsorbed on the catalyst is provided additional residence time for cracking, and a continuous stream of iluidizing gas, e.g., steam, is admitted through line 131 which also serves to strip such heavy oil and cracked product from the catalyst. 'I'he vaporous reaction product from the mild cracking operation in reactor 126 is withdrawn overhead from hopper 129 by line 132, and this line, in turn, is connected to the bottom part of fractionating tower 113. In the fractionating tower, the total vaporous feed is separated into various fractions and withdrawn as (a) gasoline and lighter product materials through an overhead line 134; (b) as a light gas oil product which is withdrawn through a line 136; (c) as a heavy oil fraction which is withdrawn from line 138; and (d) as a decanted oil which is withdrawn through a line 140. The withdrawal of these product fractions is facilitated by appropriate means such as tray 142 for the withdrawal of light gas oil, tray 144 for the withdrawal of heavy gas oil and well 146 for the withdrawal of decanted oil. Spent catalyst in reactor hopper 129 is withdrawn therefrom by means of a standpipe 150 in which there is located a slide valve 152. Air which is supplied from av source 154 ows through line 156 and steam or other inert gas is admitted through line 151, and the combined stream serves to carry the spent catalyst in transfer line 158 to regenerator 160. In line 158 and regenerator 160, the entrained light hydrocarbon content of the catalyst is burned plus a part of the coke, under conditions of relatively high velocity. The supercial linear gas velocity of the regenerator gas passing upwardly through regenerator 160 is about 3-4 feet per second, hence, all of the catalyst is carried overhead from the regenerator through a line 162, in which it flows downwardly into the upper part of a regenerator-hopper 164. The downwardly directed inlet helps to reduce the quantity of powder entrained in the gas ow to the cyclone recovery. A regenerated catalyst bed D is maintained in hopper 164, and it has a level 166. Flue gas is removed overhead from the regenerator through a line 168 and any entrained catalyst ines are separated and recovered by means of a cyclone or other suitable recovery means,

not shown. The regenerated catalyst bed D is supportedv on a grid plate 170. Below the grid plate additional combustion air is introduced by means of line 172 at the rate required in order to maintain a uid bed and to effect the desired regeneration of catalyst. Another portion of regeneration air is passed through a line 174,

and 4,this jair'stream serves to vpickup regeneratedcatalyst passing downwardly through standpipe 176, containing slide valve 178, and transporting the same through a transfer line 180 for passage through a catalyst recycle cooler 182. The catalyst withdrawn from the regenera- .tor hopper 164 is at a temperature of about 1050 F., and by means of cooler 182, the catalyst temperature is reduced to about 850 F. and it is recycled at the rate required to maintain the regenerator temperature at 1050 F. The combustion air being supplied to lines 172 and 174 is supplied by means of line 136.

With respect to catalyst replacement, the operation described above is supplied with fresh catalyst from a storage drum (not shown) `to vregenerator 230 of the severe cracking operation by means of line 231. Another stor age drum (not shown) serves as an intermediate storage vessel for the withdrawal of regenerated catalyst from regenerator 230 for supply to regenerator 164 of the mild cracking operation. Catalyst is withdrawn from the regenerator 164 and discarded from the system to maintain the desired activity level in the mild cracking operation.

Various alternative schemes can be suggested for the above operation. In one instance, the overhead product from reactor hopper 214 can be passed rst to a fractionator wherein the gasoline is separated therefrom, and this gasoline fraction alone is charged to line 120 for processing with the heavy residual oil. In another scheme, the heavy residual oil alone is combined with steam and charged directly to the mild cracking reactor 126. The heavy oil discharged from lines 111 and 140 can be recycled to the reactor 126, or these materials can be discarded from the system.

Having thus provided a description of my invention including specific examples, it is to be understood that no undue limitations or restrictions are to be imposed by reason thereof, but that the scope of the present invention is detined by the appended claims.

I claim:

l. A process for the production of gasoline which cornprises contacting a residual oil containing at least about 1 percent by weight of carbon residue and substantially free of separable gas oil suitable for high level conversion to gasoline with a diluent to vaporize a substantial portion of said residual oil prior to contact with a cracking catalyst under iluidized cracking conditions to elect during said cracking not more than about percent conversion to a reaction product containing gas oil, separating the gas oil from the reaction product and contacting the same with a cracking catalyst under cracking conditions to effect a conversion of at least about percent for the production of gasoline, said separated gas oil containing not more than about 0.6 percent carb onl residue.

2. A process for the production of gasoline which comprises contacting a residual oil containing at least about 1 percent by weight of carbon residue and substantially free of separable gas oil suitable for high level-conversion to gasoline with a diluent to vaporize a substantial portion of said residual oil prior to contact with a cracking catalyst under fluidized cracking conditions in a iirst reaction zone to effectV during cracking not more than about 20 percent conversion to a reaction product containing gas oil, separating the gas oil containing not more than about 0.6 percent carbon residue from the reaction product, contacting the separated gas oil with a cracking catalyst under cracking conditions in a second reaction zone to effect a conversion of at least about 35 percent for the production of gasoline and passing a portion of the catalyst from the second reaction zone to the first reaction zone.

3. A process for the production of gasoline which comprises contacting a residual oil containing at least about l` percent of carbon residue and at least about 0.5 p.p.m. Oimetals, nickel equivalent and substantially free of 'separable gas oil suitablefor high level conversion `to gasoline with a diluent to vaporize a substantial part of said residual oil prior to contact with a silicious catalyst under fluidized cracking conditions including a temperature of about 800 F. to about 900 F. vto elect a conversion of about 5 to about 20 percent to produce a reaction product containing gas oil substantially free of carbon residue and metal contaminants, separating the gas oil from the reaction product and contacting the separated gas oil with a silicious catalyst under uidized cracking conditions including a temperature of about 800 F. to about 1050 F. to effect a conversion of about r35to about 70 percent for the production of a high antiknock quality gasoline.

, 4. A process for the production of gasolinewhich comprises contacting a residual oil containing at least about l percent by weight of carbon residue, at least about.0.5 p.p.m. of metals, nickel equivalent, andsubstantially free of separable gas oil suitable for high level conversion to gasoline with a diluent to vaporize a substantial portion of said residual oil prior to contact with a uidized mass of iinely divided silicious catalyst having an average D-l-L activity of about l0 to about 25 in a first cracking zone under suitable cracking conditions including a temperature of about 700 F. to about 1000" F. to elect a conversion of about 5 to about 20 percent such that a reaction product containing gas oil substantially free .of metal contaminants and carbon residue is obtained, separating the gas oil from the reaction product, contacting the separated gas oil with a iludized mass of finely divided siliceous catalyst having an average D-i-L activity of about 20 to about 45, said activity being substantially greater than the activity maintained in the rst cracking zone, under suitable iluidized cracking conditions iucludinga temperature of about 800 to about 10.50 F. to effect a conversion of about 3S to about 70'percent for the production of a high anti-knock quality gasoline, and passing a portion of the catalyst from the second cracki@ zone to the rst cracking zone in a quantity sufficient to maintain the desired D-i-L activity level therein.

5. A process for the production of gasoline which com-` prises contacting a residual oil containing at least about l percent by weight of carbon residue, at leastabout 0.5 p.p.m. of metals, nickel equivalent, and substantially free of separable gas oil suitable for high level conversion to gasoline with a diluent to vaporize a substantial portion of said residual oil prior to contact with a tluidized mass of iinely divided siliceous catalyst having an average D-i-L activity of about 10 to about 20 in a first cracking zone under suitable cracking conditions to effect a conversion of about 5 to about 20 percent such that a reaction product containing a gas oil substantially free of carbon residue and metal contaminants is obtained, separating the gas oil from the reaction product, contacting the separated gas oil with a tluidized mass of tinely divided siliceous catalyst having an average D|L activity of about 30 to about 36 in a second cracking zone under suitable cracking conditions to eiect a conversion of about 35 to about 70 percent for the production of a high anti-knock quality gasoline product, and passing a portion of the catalyst utilized for cracking in the second cracking zone to the rst cracking zone in a quantity sucient to maintain the desired D-i-L activity therein.

6. A process for the production of gasoline whichcomprises contacting a residual oil containing at least about l percent of carbon residue, at least about 0.5 p.p.m. of metals, nickel equivalent, and substantially free of separable gas oil suitable for high level conversion to gasoline with a diluent to vaporize a substantial portion of said residual oil prior to contact with a siliceous Ycatalyst having an average D-t-L activity of about 10 to about 25 under iluidized conditions at a temperature of about 700 to about l000 F., at a weight space velocity of about 0.1 to about l5, a catalyst to oilv ratio of about 0.1 to" about 2.0,. a pressure of about l atmosphere to about'50' -assaars p.s.i.g., said operating conditions being selected toeiect a conversion of about 5 to about 20 percent in order to produce a reaction product containing gas oil substantially free of carbon residue and metal contaminants, separating the gas oil from the reaction product, contacting the separated gas oil with a siliceous catalyst having a D-l-L activity of about 20 to about 45, at a temperature of about 800 to about 1025 F., a weight space velocity of about 0.1 to about 10, a catalyst to oil ratio of about 0.5 to about 25, a pressure of about 1 atmosphere to about 50 p.s.i.g., and said conditions being selected to effect a conversion of about 35 to about 70 percent t0 produce a gasoline product of high anti-knock quality.

7. process for the production of gasoline which comprises contacting a residual oil containing a carbon residue of about 2.5 to about 30 percent, at least about 2 p.p.m. of metals, nickel equivalent, and substantially free of separable gas oil suitable for high level conversion to gasoline with a diluent to vaporize a substantial portion of said residual oil prior to contact with a siliceous catalyst having an average D-l-L activity of about 20 to about 20 under uidized conditions at a temperature of about 800 to about 900 F., a weight space velocity of about 1 to about 10, a catalyst to oil ratio of about 0.5 to about 5.0, a pressure of about 1 atmosphere to about l5 p.s.i.g., said conditions being selected to effect a conversion of about 5 to about 20 perecent such that a reaction product containing gas oil substantially free of carbon residue and metal contaminants is obtained, separating the gas oil from the reaction product, contacting the separated gas oil with a uidized siliceous catalyst having an average D-{L activity of about 30 to about 36, at a temperature of about 900 to about 1000 F., a pressure of about 1 atmosphere to about l5 p.s.i.g., a catalyst to oil ratio of about 2 to about 10, and said conditions being selected to effect a conversion of about 45 to about 60 percent such that a gasoline product of high anti-knock quality is produced.

8. A process for the production of gasoline which comprises subjecting a crude oil to treatment in a separation zone for the production of a first gas oil fraction and a reduced crude fraction containing about 2.5 to about 30 percent of carbon residue and at least 2 p.p.m. of metals, nickel equivalent, contacting the reduced crude with a hydrocarbon diluent to vaporize a substantial portion of said reduced crude fraction prior to contact with a siliceous catalyst in a tirst cracking zone under uidized conditions suitable to effect a conversion of about 5 to about 20 percent such that a reaction product containing gas oil substantially free of carbon residue and metal contaminants is produced, passing the reaction' product to the aforesaid separation zone wherein the gas oil product is separated and combined with the first gas oil fraction and the product boiling higher than gas oil is combined with the reduced crude for recycle to the rst cracking zone, contacting the total gas oil material with a siliceous catalyst in a second cracking zone under conditions suitable to effect a conversion of about 35 to about 70 percent such that a gasoline product of high anti-knock quality is produced.

9. A process for the production of gasoline which comprises contacting a residual oil containing about 2.5 to about 30 percent of carbon residue, at least 2 p.p.m. of metals, nickel equivalent, and substantially free of separable gas oil suitable for high level conversion to gasoline with a hydrocarbon diluent to vaporize a substantial portion of said residual oil prior to contact with a siliceous catalyst having an average D-i-L activity of about l to about 25 in a tirst cracking zone under suitable cracking conditions to effect a conversion of about to about 20 percent such that a reaction product containing gas oil substantially free of carbon residue and metal contaminants is produced, separating the gas oil from the reaction product, contacting the separated gas oil with a siliceous catalyst having an average DV-l-L activity of about 20 to about 45 and which is greater than the activity of the catalyst in the iirst cracking zone under suitable tiuidized cracking conditions in the second cracking zone to effect a conversion of about 35 to about 70 percent such that the catalyst becomes contaminated with carbonaceous material and a gasoline product of high anti-knock quality is produced, passing a portion of contaminated catalyst from the second cracking zone to a regeneration zone wherein the catalyst is regenerated by combustion with an oxygen containing gas, passing a portion of regenerated catalyst to the rst cracking zone and withdrawing catalyst in a similar quantity therefrom in order to maintain the desired activity therein.

10. A process for the production of gasoline which comprises subjecting a crude oil to treatment in a separation zone for the production of a rst gas oil fraction and a reduced crude fraction, subjecting the reduced crude fraction to distillation under vacuum to produce a second gas oil fraction and a residual oil containing about 2.5 to about 30 percent by weight of carbon residue and about 5 to about 70 p.p.m. of metals, nickel equivalent, contacting the residual oil with a hydrocarbon diluent to vaporize a substantial portion of said residual oil prior to contact with a siliceous catalyst in a iirst cracking zone under fluidized cracking conditions to effect a conversion of about 5 to about 20 percent such that a reaction product containing gas oil substantially free of carbon residue and metal contaminants is produced, separating a third gas oil fraction from the reaction product, combining the first, second and third gas oil fractions, said combined gas oil fractions having a carbon residue of not more than 0.6 percent and not more than 2 p.p.m. of metals, nickel equivalent, contacting the combined gas oil with a siliceous catalyst in a second iiuidized cracking zone under suitable cracking conditions to effect a conversion of about 35 to about 75 percent such that a gasoline product of high antiknock quality is produced.

1l. A process for the production of gasoline which comprises treating a reduced crude in a separation zone for the production of a rst gas oil fraction and a residual oil fraction, contacting the residual oil containing about 2.5 to 30 percent carbon residue, about 5 to 70 p.p.m. of metals, nickel equivalent, and substantially free of separable gas oil which is suitable for high level conversion of gasoline with the products of the second cracking step hereinafter described to vaporize a substantial portion of said residual oil prior to contact with a siliceous catalyst in a first fluidized cracking zone having an average D-l-L activity of about 10 to 25 and under suitable tiuidized cracking conditions to effect a conversion of about up to about 20 percent such that the catalyst is contaminated with carbonaceous material and a reaction product containing gas oil substantially free of carbon residue and metal contaminants is produced, subjecting the reaction product to treatment in a second separation zone whereby a gasoline product, a second gas oil fraction and a third fraction of liquid product boiling above the gas oil are produced, combining the said third fraction with the residual oil being charged to the rst cracking zone, combining the rst and second gas oil fractions and charging the same to a second iiuidized cracking zone containing a siliceous catalyst having an average D-l-L activity of about 20 to about 45 such that a reaction product containing gasoline of high anti-knock quality is produced, and combining the reaction product from the second cracking zone with the residual oil and the third fraction being charged to the first reaction zone.

12. A process for the production of gasoline which comprises contacting a residual oil containing at least about l percent by weight of carbon residue in the presence of a diluent to vaporize a substantial portion of said residual oil prior to contact with a tiuidized cracking catalyst under cracking conditions in a rst zone to effect not more than about 20 percent conversion to a reaction productcontaining gas oil, `separating the gas oil from the reaction product and passing the separated gas oil to a second zone wherein it is passed in contact with a cracking catalyst under uidized cracking conditions to effect a conversion of at least about 35 percent to produce a second vaporous reaction product containing gasoline and combining the second vaporous product as the diluent for the residual oil passed to said first zone.

13. The process of claim 12 wherein the residual oil oil is a heavy residual oil having an API gravity of about 1 to about 12 and containing about 2.5 to about 30 percent by weight of carbon residue.

14. A process for the production of gasoline which comprises contacting a gas oil with a uidized cracking catalyst in a iirst zone under uidized cracking conditions to eiect a conversion of at least about 35 percent, there by producing a vaporous reaction product containing gasoline, gas oil, and product material heavier than gas oil, combining said vaporous reaction product at an elevated temperature with a residual oil to vaporizea substantial portion thereof and passing the same to a second zone in contact with a iiuidized cracking catalyst under less severe cracking conditions to effect not more than about 20 percent conversion to produce a reaction product containing gasoline, gas oil and product material heavier and lighter than said gas oil, passing the total eluent from the second zone to a separation zone to recover a gas oil fraction and a heavier than gas oil fraction, passing the gas oil fraction from the separation zone to the rst zone and passing the heavier than gas oil fraction from the separation zone tio the second zone.

15. A process for the production of gasoline which comprises contacting a residual oil containing at least about 1 percent by Weight of carbon residue and substantially free of separable gas oil suitable for high level conversion to gasoline in the presence of a diluent to vaporize ay substantial portion of said residual oil prior to contact with a cracking catalyst in a first cracking zone under iluidized conditions to eiect not more than about 20 percent conversion to a reaction product containing gas oil, separating a gas oil product from the rst reaction product and passing the gas oil to a second zone wherein the gas oil is passed in contact with a cracking catalyst to produce a vaporous reaction product containing gasoline thereby contaminating the catalyst with carbonaceous material, withdrawing contaminated catalyst from the second zone and passing the contaminated catalyst to a regeneration zone wherein carbonaceous material is burned to produce a uc gas, withdrawing regenerated catalyst from the regeneration zone and pass- 15 ing separate portions thereof to the first and second cracking zones and employing a portion of the ue gas as the 'diluent for the residual oil passed to the first cracking zone.

16. A process for the production of gasoline which comprises contacting a residual oil containing at least about l percent by weight of carbon residue with a cracking catalyst in a first zone under uidized cracking conditions to elect not more than about 20 percent conversion and thereby producing a reaction product containing gas oil, separating the gas oil from the reaction product and passing the gas oil to a second zone where inrit is contacted with a cracking catalyst to produce a second vaporous reaction product containing normally gaseous product material and gasoline and contaminating the catalyst with carbonaceous material, withdrawing the contaminated catalyst from the second zone and passing the contaminated catalyst to a regeneration zone wherein carbonaceous material is burned to produce flue gas, withdrawing regenerated catalyst from the regeneration zone and passing separate portions thereof to the first and second zones and combining with the residual oil prior to contact with catalyst with a diluent to vaporize a substantial portion of said residual oil selected from the group consisting of flue gas from the regeneration zone, the second vaporous reaction product and the normally gaseous product material.

17. The process of claim 16 wherein the residual oil is a heavy residual oil having an API gravity of about 1 to 12 and containing 2.5 to 30 percent by weight of carbon residue.

References Cited in the le of this patent UNITED STATES PATENTS 2,286,447 Thomas lune 16, 1942 2,360,553 Egloi Oct. 17, 1944 2,378,531 Becker June 19, 1945 2,414,883 Martin Ian. 28, 1947 2,636,844 Kimberlin et al Apr` 28, 1953 2,641,573 Weikart lune 9, 1953 2,685,559 Voorhies Aug. 3, 1954 2,737,474 Kimberlin Mar. 6, 1956 2,766,184 Blanding Oct. 9, 1956 OTHER REFERENCES Wrightson: Determination of Traces of Iron, Nickel and Vanadium in Petroleum Oils, Analytical Chemistry, vol. 21, No. 12, December 1949, page 1545.

Mills: Aging of Cracking Catalysts, Ind. Eng. Chem., vol. 42, No. 1, January 1950, pages 184, 185.

UNITED STATES PATENT OFFICE CERTIFICATE 0F CQRRECTION Patent' No. 2,882,218 Aprii 14, 1959 'Joseph W. Jewell It ie hereby certifiedV that error appears in the printed specification of the' above numbered patent requiring correction and that the' said Letters Patent should read as corrected below.

Column 8, line '72, for "ih it preheatedn 'read v-'- in it is preheated d lColumn 13, line 22, for "2O to about 20" read lO to about 2O line 2 signed and ls881881 this 25th day of August 1959.

Y (SEALS Attest:

KARLHj AXLINE ROBERT c. wATsoN b-'besvtingj Officer y Commissioner of Patents 

1. A PROCESS FOR THE PRODUCTION OF GASOLINE WHICH COMPRISES CONTACTING A RESIDUAL OIL CONTAINING AT LEAST ABOUT 1 PERCENT BY WEIGHT OF CARBON RESIDUE AND SUBSTANTIALLY FREE OF SEPARABLE GAS OIL SUITABLE FOR HIGH LEVEL CONVERSION TO GASOLINE WITH A DILUENT TO VAPORIZE A SUBSTANTIAL PORTION OF SAID RESIDUAL OIL PRIOR TO CONTACT WITH A CRACKING CATALYST UNDER FLUIDIZED CRACKING CONDITIONS TO EFFECT DURING SAID CRACKING NOT MORE THAN ABOUT 20 PERCENT 